How Tiny Germs Trick the Immune System

Have you ever wondered how some germs manage to hide from the body's defenses? Mycoplasma mycoides subsp. capri (Mmc) is a tiny, cell wall-less bacterium that can cause disease in goats and cattle. It has a clever trick: it can switch on and off a sugar-like coating called capsular polysaccharide (CPS). This switch affects how the immune system reacts to the germ. When Mmc has its CPS coating, it becomes a master of disguise. It can sneak past immune cells without causing much of a fuss. In fact, when blood cells from goats and cattle were exposed to Mmc with its CPS coating, they barely reacted. The germ could even survive inside certain immune cells, called monocyte-derived macrophages (MDMs), which helps it spread and stay hidden in the body. But when Mmc loses its CPS coating, it exposes other parts of itself, like surface proteins and lipoproteins. This change makes the immune system sit up and take notice. Blood cells exposed to the uncoated Mmc showed signs of distress, like increased cell death. The uncoated Mmc also suppressed the expression of major histocompatibility complex (MHC) on antigen-presenting cells and triggered the release of pro-inflammatory cytokines and chemokines. These are the same signs seen in infected animals. Moreover, the uncoated Mmc caused inflammatory cell death in MDMs. This ability to switch its CPS coating allows Mmc to manipulate the immune system. It can choose to lay low and persist in the body or trigger a strong immune response that leads to disease. This makes Mmc a formidable opponent for the immune system to tackle.